1. Field of the Invention
This invention relates to devices and methods for the determination of the concentration of an analyte in a human tissue. More specifically, this invention relates to devices and methods for the non-invasive determination of the concentration of one or more analytes in vivo in a human tissue, wherein an optical property at a given depth in the tissue is significantly affected by a given analyte.
2. Discussion of the Art
Non-invasive monitoring of analytes in the human body by optical devices and methods is an important tool for clinical diagnosis. xe2x80x9cNon-invasivexe2x80x9d (alternatively referred to herein as xe2x80x9cNIxe2x80x9d) monitoring techniques measure in vivo concentrations of analytes in the blood without taking out a blood sample from the human body. As defined herein, a xe2x80x9cnon-invasivexe2x80x9d technique is one that can be used without removing a sample from, or without inserting any instrumentation into, the human body. The ability to determine an analyte, or a disease state, in a human subject without performing an invasive procedure, such as removing a sample of blood or a biopsy specimen, has several advantages. These advantages include ease in performing the test, reduced pain and discomfort to the patient, and decreased exposure to potential biohazards. These advantages will promote increased frequency of testing, accurate monitoring and control of a disease condition, and improved patient care. Representative examples of non-invasive monitoring techniques include pulse oximetry for oxygen saturation (U.S. Pat. Nos. 3,638,640; 4,223,680; 5,007,423; 5,277,181; 5,297,548). Another example is the use of laser Doppler flowmetry for diagnosis of circulation disorders (Tooke et al, xe2x80x9cSkin microvascular blood flow control in long duration diabetics with and without complicationxe2x80x9d, Diabetes Research, Vol. 5, 1987, pages 189-192). Other examples of NI techniques include determination of tissue oxygenation (WO 92/20273), determination of hemoglobin (U.S. Pat. No. 5,720,284), and hematocrit (U.S. Pat. Nos. 5,553,615; 5,372,136; 5,499,627; WO 93/13706). Determination of bilirubin was also described in the art (R. E. Schumacher, xe2x80x9cNoninvasive measurement of bilirubin in the newbornxe2x80x9d, Clinics in Perinatology, Volume 17, 1990, pages 417-435, and U.S. Pat. No. 5,353,790).
Measurements in the near-infrared region of the electromagnetic spectrum have been proposed, or used, in the prior art. The 600 nm to 1300 nm region of the electromagnetic spectrum represents a window between the visible hemoglobin and melanin absorption bands and the strong infrared water absorption bands. Light having a wavelength of 600 nm to 1300 nm can penetrate sufficiently deep into the skin to allow use thereof in a spectral measurement or a therapeutic procedure.
Oximetry measurement is very important for critical patient care, especially after the use of anesthesia. Oxygenation measurements of tissue are also important diagnostic tools for measuring oxygen content of the brain of the newborn during and after delivery, for monitoring tissue healing, and in sports medicine.
Non-invasive determination of hemoglobin and hematocrit values in blood would offer a simple, non-biohazardous, painless procedure for use in blood donation centers. Such techniques could increase the number of donations by offering an alternative to an invasive procedure, which is inaccurate and may possibly lead to the rejection of a number of qualified donors. Non-invasive determination of hemoglobin and hematocrit values would be useful for the diagnosis of anemia in infants and mothers, without the pain associated with blood sampling. Non-invasive determination of hemoglobin has been considered as a method for localizing tumors and diagnosis of hematoma and internal bleeding (S. Gopinath, et al., xe2x80x9cNear-infrared spectroscopic localization of intracamerial hematomasxe2x80x9d, J. Neurosurgery, Vol. 79, 1993, pages 43-47). Non-invasive determination of hematocrit values can yield important diagnostic information on patients with kidney failure before and during dialysis (R. R. Steuer, et al., xe2x80x9cA new optical technique for monitoring hematocrit and circulating blood volume; Its application in renal dialysisxe2x80x9d, Dialysis and Transplantation, Volume 22, 1993, pages 260-265). There are more than 50 million dialysis procedures performed in the United States and close to 80 million dialysis procedures performed world-wide annually.
Non-invasive diagnosis and monitoring of diabetes may be the most important potential advantage for non-invasive diagnostics. Diabetes mellitus is a chronic disorder of carbohydrate, fat, and protein metabolism characterized by an absolute or relative insulin deficiency, hyperglycemia, and glycosuria. At least two major variants of the disease have been identified. xe2x80x9cType Ixe2x80x9d accounts for about 10% of diabetics and is characterized by a severe insulin deficiency resulting from a loss of insulin-secreting beta cells in the pancreas. The remainder of diabetic patients suffer from xe2x80x9cType IIxe2x80x9d, which is characterized by an impaired insulin response in the peripheral tissues (Robbins, S. L. et al., Pathologic Basis of Disease, 3rd Edition, W. B. Saunders Company, Philadelphia, 1984, p. 972). If uncontrolled, diabetes can result in a variety of adverse clinical manifestations, including retinopathy, atherosclerosis, microangiopathy, nephropathy, and neuropathy. In its advanced stages, diabetes can cause blindness, coma, and ultimately death.
The concept upon which most NI detection procedures are based involves irradiating a tissue or a vascular region of the body with electromagnetic radiation and measuring the spectral information that results from at least one of three primary processes: absorption, scattering, and emission. The extent to which each of these processes occurs is dependent upon a variety of factors, including the wavelength of the incident radiation and the concentration of analytes in the body part. Signals are measured as a change in reflectance or transmittance of the body part. Concentration of an analyte, e.g., glucose, hemoglobin or bilirubin is determined from the spectral information by comparing the measured spectra to a calibration data set. Alternatively the concentration of an analyte is determined by comparing the magnitude of the change in signal to the results of calculations based on a physical model describing the optical properties of the tissue under examination. Various categories of non-invasive measurement techniques will now be described.
NI techniques that utilize the interaction of a sample with infrared radiation can be categorized according to three distinct wavelength regions of the electromagnetic spectrum: near-infrared (NIR), mid-infrared (MIR) and far-infrared (FIR). As defined herein, NIR involves the wavelength range from about 600 nm to about 1300 nm, MIR involves the wavelength range from about 1300 nm to about 3000 nm, and FIR involves the wavelength range from about 3000 nm to about 25000 nm. As defined herein, xe2x80x9cinfraredxe2x80x9d (or IR) is taken to mean a range of wavelengths from about 600 nm to about 25000 nm.
Due to the highly scattering and absorption nature of the human skin and tissue, light in the 600 nm to 1300 nm spectral range penetrates the skin and underlying tissues to different depths. The tissue depth at which most of the reflectance signal is generated (sampling depth) depends on the wavelength of light and positioning of the source and detector. Analyzing the reflected or transmitted signal without accounting for the effect of different layers of skin can lead to erroneous estimates of the optical properties of the tissue and hence, the concentration of metabolites determined from these measured properties. The stratum corneum, epidermis, dermis, adipose tissue, and muscle layers can interact with light differently and contribute separately to the measured signals. Controlling the sampling depth of the light and understanding the effect of the different layers of the skin on the generated signal are important for the accurate non-invasive determination of metabolites in tissues. The NIR spectral region has been used for determination of blood oxygen saturation, bilirubin, hemoglobin, hematocrit, and tissue fat content. It is also used for exciting and detecting therapeutic agents in photodynamic therapy. At longer wavelengths in MIR region, water absorption bands are dominant in tissue spectra. There are some narrower spectral windows in the 1500 nm to 1900 nm range and the 2100 nm to 2500 nm range, where both in vitro and in vivo tissue measurements have been performed.
Light striking a tissue will undergo absorption and scattering. Most of the scattered photons are elastically scattered, i.e., they keep the same frequency as the incident radiation (e.g., Rayleigh scattering). A small fraction of the scattered light (less than one in a thousand incident photons) is inelastically scattered (Raman scattering). Unless otherwise indicated herein, xe2x80x9cscatteringxe2x80x9d refers to elastic scattering.
Because of the multiple scattering effect of tissue, optical measurements of either transmission or reflectance will contain tissue scattering information, as well as absorption information. Tissue scattering information includes cell size and cell shape, depth of the tissue layer in which scattering occurs, and refractive index of intracellular fluids and extracellular fluid (interstitial fluid). Absorption information includes absorption by tissue components, such as hemoglobin, melanin, and bilirubin, and the overtone absorption of water, glucose, lipids, and other metabolites.
One method for measuring elastic light scattering of tissues and turbid media is spatially resolved diffuse reflectance (SRDR), where detection fibers are placed at multiple distances from a light entry point. Reflectance values at different distances from the illumination point are used to calculate the absorption and scattering coefficients of the tissue based on photon diffusion theory models or numerical calculations such as Monte Carlo simulations. The values of the absorption and scattering coefficients are then used to correlate with the concentration of an analyte.
As shown in FIG. 1, light is introduced into the surface of a tissue sample, such as a body part, at an introduction site. The diffusely reflected light is measured at two or more detection sites located on the surface of the sample (e.g., the skin) at different distances, r, from the introduction site. The dependence of the intensity of the diffusely reflected light, i.e., reflectance R, as a function of the distance between the detector and the light source in touch with the sample (r) is used to derive scattering and absorption coefficients of the tissue sample. These coefficients, in turn, are correlated with the concentration of analyte(s) (see, for example, U.S. Pat. No. 5,492,118).
European Patent No. 0843986A2 describes a reflectance spectrophotometer for blood glucose measurement from human skin. The spectrophotometer intends to minimize the influence of undesirable spectral information from the epidermis by separating the light introduction site and the light detection site. This undesirable spectral information is in the form of diffuse surface reflectance that depends on the condition of the surface of the skin. In the arrangement disclosed therein, however, light penetrates through the epidermis twicexe2x80x94once at the light introduction site and once at the light detection site, and its properties will be affected by the optical properties of the epidermis. The method of European Patent No. 0843986A2 is based on the erroneous assumption that light penetrating to a lower layer of the skin will not be affected by the optical properties of the upper layers. The method does not account for both of the scattering and absorption properties of different skin layers being affected by different tissue analytes and relies mainly on absorption of glucose in the 1300-2500 spectral range, which is dominated mainly by water absorption.
The above prior art methods do not address the effect of skin layers on signal, distribution of analytes in these layers, and the effect of each analyte on the optical properties of each layer.
The use of absorption and scattering coefficients derived from mathematical models that assume homogeneous non-layered structures can lead to inaccurate determination of analytes in tissue. Further, use of measurement methods that average out over several layers and multiple compartments of the skin or other samples can also lead to complicated and misrepresenting data.
An important variable in an in vivo measurement is the fluctuation of blood volume at the measurement site. Fluctuation in blood volume at the measurement site could result from such factors as lack of anatomical homogeneity, blood vessel dilation or constriction due to hormonal control, or change in ambient temperature. A change in the volume fraction of the blood can lead to erroneous measurement if the concentration of a non-absorbing analyte is calculated from scattering data as suggested by U.S. Pat. Nos. 5,551,422 and 5,492,118. Scattering of red blood cells and the effect of blood volume on fluid contents of tissue affect the values of the scattering coefficients and hence the calculated concentration of analytes such as glucose determined in the near-IR (600-nm to1300 nm). In the same manner, changes in scattering values of tissue affect the calculated values of the absorption coefficient and can affect the calculated concentrations of absorbing analytes, such as hemoglobin, bilirubin, and colored therapeutic agents.
Although a variety of techniques have been disclosed in the art, there is still no commercially available device that provides non-invasive glucose measurements with an accuracy that is comparable to the established invasive methods. Devices for non-invasive measurement of bilirubin and hematocrit have been commercialized. However, signals obtained by prior art methods operate on the assumption that the tissue comprises a single uniform layer. As the change in optical signal due to a weakly absorbing analyte such as glucose is expected to be small, any approximation in the over-simplified skin model or in the calculation of the scattering and absorption coefficients will lead to erroneous results. The signals, for example, are vulnerable to the effects of top layers of the skin, which are significantly different from the deeper layers of the skin in terms of textures, colors, and other properties.
Thus, there is a continuing need for improved NI instruments and methods that are unaffected by variations in skin structures and layers or account for the effect of skin layers. There is also a need for instruments with simple calibration schemes that can be set in the factory and periodically checked for accuracy in the field.
Co-pending U.S. application Ser. No. 09/198,049, filed Nov. 23, 1998 (xe2x80x9cNon-invasive sensor capable of determining optical parameters in a sample having multiple layersxe2x80x9d), assigned to the assignee of this application, describes methods for determining optical properties of tissue with multiple layers. The methods involve the use of multiple groups of closely spaced optical fibers that are located at spatially resolved measurement sites. Each group yields information on a specific layer in the sample that is determined by the distance between the light illumination site and the residing site of the group. The layers described in the co-pending application are within the depth of 3 mm for human tissue samples. In body parts with a thin skin such as the forearm or the abdomen, this depth encompasses the stratum corneum, the epidermis and the dermis layers.
Skin components affect its optical properties in different ways depending if they are strongly absorbing, such as hemoglobin, bilirubin and melanin, or strongly scattering such as cells and muscle fibers. The color of the human skin is affected mostly by the contents of hemoglobin, melanin and bilirubin. Densities, sizes and shapes of cells and the refractive indexes of intercellular fluids (interstitial fluid) and intracellular fluid will affect skin scattering, especially in the relatively uniform epidermis and upper dermis. Analytes that may cause changes in the cell sizes and shapes and the refractive indexes of fluids can be tracked by measuring the scattering coefficient of these layers. Compounds that may have significant effect on these changes in the interstitial fluid are glucose, salts, proteins, fatty acids, and water. However, as light gets deeper into the dermis it starts to probe capillary beds and upper and lower plexus. Further deeper in the subcutaneous tissues, light interacts with capillaries, veins, various corpuscles, adipose tissues, etc.
We have discovered that the measurement of trans-cutaneous diffuse reflectance at a single sampling distance can achieve good correlation with the concentration of an analyte in a biological sample, such as, for example, human tissue. Such correlation has been found to depend on the sampling distance and reaches an optimal result at a defined sampling distance for a given analyte and a given biological sample.
This invention provides a method for determining the concentration of an analyte in a biological sample, typically one having a plurality of layers, e.g., a sample of human tissue. The method comprises the steps of:
(a) introducing a beam of light into the biological sample at a light introduction site on a surface of the biological sample;
(b) collecting the light re-emitted from the biological sample at a light collection site on the surface of the biological sample, the light collection site located at a distance from the light introduction site, the distance of the light collection site from the light introduction site corresponding to a sampling depth in the biological sample, at which sampling depth an optical property of the biological sample is significantly affected by the analyte;
(c) determining the intensity of the collected light; and
(d) determining the concentration of the analyte from the intensity of the collected light.
The method involves measuring the light re-emitted at a distance from the light introduction site and correlating the intensity of the re-emitted light to the concentration of an analyte. For a given biological sample, the distance between the light collection site and a light introduction site (i.e., sampling distance) corresponds to the depth from the surface into the biological sample at which scattering and absorption events significantly affect the intensity of re-emitted light (i.e., sampling depth). Prior knowledge about the biological sample determines the optimal sampling depth for performing a measurement for a specific analyte and the corresponding sampling distance needed to reach that optimal sampling depth. Optimization of the sampling distance, as well as the correlation relationship, can be established in a calibration procedure described herein.
In a preferred embodiment of this invention, a method for determining the concentrations of a plurality of analytes in a biological sample, typically one having a plurality of layers, e.g., a sample of human tissue, comprises the steps of:
(a) introducing a beam of light into the biological sample at a light introduction site on a surface of the biological sample;
(b) collecting the light re-emitted from the biological sample at a light collection site on the surface of the biological sample, the light collection site located at a distance from the light introduction site, the distance of the light collection site from the light introduction site corresponding to a sampling depth in the biological sample, at which depth an optical property of the biological sample is significantly affected by one analyte of the plurality of analytes;
(c) determining the intensity of the collected light;
(d) determining the concentration of the one analyte of the plurality of analytes from the intensity of the collected light; and
(e) repeating steps (a), (b), (c), and (d) for at least another analyte of the plurality of analytes.
The method of this invention is applicable for an arrangement wherein a single light introduction site and one or more light collection sites are employed. The method of this invention is also applicable for an arrangement wherein a single light collection site and one or more light introduction sites are employed. In either variation, the method is capable of determining the concentration of at least one component of a sample of human tissue having a plurality of layers, wherein each of these layers has different properties that are affected differently by the concentration of analytes in the tissue.
Another aspect of this invention involves a method whereby the selection of the sampling distance at which each analyte is determined is accomplished automatically by means of a programmable device. At the time of measurement, the sampling distance and the wavelength(s) of the incident light are selected by a computer, based on an input that includes the specific analyte to be determined and the prior knowledge about the sample.
In another aspect, this invention provides an apparatus for determining the concentration of at least one analyte in a biological sample, typically one having a plurality of layers, e.g., a sample of human tissue. The apparatus comprises:
(a) a means for introducing a beam of light into the biological sample at a light introduction site on a surface of the biological sample;
(b) a means for collecting light re-emitted from the biological sample at at least one light collection site on the surface, the at least one light collection site located at a predetermined sampling distance from the light introduction site, the predetermined sampling distance corresponding to a sampling depth, at which sampling depth an optical property of the biological sample is significantly affected by the analyte;
(c) a means for determining the intensity of the light collected at each light collection site; and
(d) a means for determining the concentration of the at least one analyte from the intensity of the light collected at one of the light collection sites.
In an alternative of this apparatus, the apparatus comprises:
(a) a means for introducing a beam of light into the biological sample at at least one light introduction site on a surface of the biological sample;
(b) a means for collecting the light re-emitted from the biological sample at a light collection site on the surface, the at least one light introduction site being located at a predetermined distance, as measured on the surface, from the light collection site, each predetermined distance corresponding to a predetermined sampling depth in the biological sample;
(c) a means for determining the intensity of the light collected at the light collection site; and
(d) a means for determining the concentration of at least one analyte from the intensity of the light collected at the light collection site.
In another aspect, a non-stationary illumination and detection system can be used and the sampling distance can be selected by moving a single illuminating element on the skin surface via a mechanism similar to a compact disk (CD) player read head. With a single light collecting element fixed at a given light collection site, the illuminating element can be moved to a predetermined position and thereby illuminate a site on the skin surface that is at a desired distance from the light collection site. Mechanisms for directing a light beam to predetermined sampling distances include beam steering devices such as moving mirrors or prisms. Alternatively, a system can comprise a stationary illuminating element and a movable light collection element.
This invention provides the following advantages over techniques that use a spatially resolved diffuse reflectance measurement (U.S. Pat. Nos. 5,075,695; 5,492,118; and 5,551,422):
(1) This invention accounts for the effect of the layers of tissue samples on the measurement.
(2) Selection of sampling distance, and, hence sampling depth, allows collection of optimal analyte signal relative to interfering signal for each analyte and each individual.
(3) This invention incorporates both absorption and scattering information and allocates appropriate balance between both types of information to maximize the effectiveness of analyte determination.
(4) In the normal mode of operation of this invention, signal detection relies on measurement at only one sampling distance, thereby simplifying the instrumentation.
(5) The method of this invention directly correlates the intensity of light collected to the concentration of an analyte and consequently eliminates the need for an algorithm for handling results based on assumptions such as the diffusion theory approximation or the complex Monte Carlo modeling computation. This invention also eliminates the errors associated with the conversion of reflectance values to scattering and absorption coefficients through empirical or semi-empirical algorithms.